Introduction to home theater audio

                                          Introduction to home theater audio

Nowadays, home speakers have a very high penetration rate, and audio knowledge is still too professional for many new friends. First-time home audio players need to understand the following basic knowledge of home audio.

1. Classification of speakers

(1) Closed type and inverted type

Closed features: poor low frequency, simple and easy to do, low distortion, suitable for listening to classical music, slightly lower sensitivity;

Inverted phase features: good low frequency, high sensitivity, good instantaneous characteristics, complex design and debugging.

(2) floor type and bookshelf type

Floor-standing (big speaker) features: high, medium and low frequency balance, good low frequency, requires a large space, poor sense of positioning, comprehensive and real performance, high price.

Bookshelf features: good positioning performance, excellent mid-high frequency, insufficient bass, suitable for small spaces.

(3) Two-way and three-way frequency

Reason for frequency division: It is difficult to make full-range speakers.

The two-way frequency division method has higher requirements on the speaker than the three-way frequency division method, and the intermodulation distortion is large, but the debugging is simple and the price is low.

(4) Active and passive

Active means that the box contains a power amplifier, while passive is just the opposite.

(5) Box material

The box material should have considerable stiffness and internal damping attenuation characteristics. There are mainly original wood boards, machine-made boards and stone materials. Among them, the original wood board is of good quality but the cost is high; machine-made boards include high, medium and low-fiber boards, particle boards, fiberboards, multi-layer boards, etc., and high-fiber boards have good effects but are rare; MDF is the most widely used machine-made board; The appearance of fiberboard is easy to be confused with MDF, but the quality is much worse (easy to deform when exposed to moisture, poor rigidity); particleboard is easily deformed when exposed to moisture, but it is low in price and widely used; multilayer board (also known as plywood) is not as good as MDF, But better than fiberboard and particleboard. Stone speakers have a good sense of layering and presence, and the bass is full and flexible, but it needs to be matched with sound-absorbing materials with a large coefficient.

(6) Sensitivity

An index to measure the electroacoustic conversion efficiency of a loudspeaker. Refers to the sound pressure value at a distance of 1 meter from the speaker when 1W of electric power is applied, and the unit is "dB". The higher the sensitivity, the lower the power requirement for the power amplifier. The sensitivity is 3dB lower, and the power of the power amplifier needs to be doubled. But the sensitivity is too high, and the distortion is difficult to do well.

(7) (rated) input power

Rated input power: The maximum input power allowed for long-term continuous normal use. In addition, the maximum input power refers to the maximum input power that can be tolerated for a short time.

Nominal Impedance: The impedance of the speaker when the input signal is 400Hz.

Frequency Response: Width and flatness.

Directivity: The intensity of sound in different directions and angles.

Distortion: Distortion should be less than 5%.

Recommended brands: Swans Audio Hivi, Boss Audio Bose, JBL Audio, Pioneer Audio, Shanshui Audio, etc. (for reference only)

Second, the components of the home theater

Similar to the components of a movie theater, it is mainly composed of three parts: audio-visual room, film and television equipment, and auxiliary facilities. The film and television equipment mainly includes AV power amplifiers, speakers, large-screen TVs, audio and video signal sources, etc.

Power amplifier: including source selection, signal processing, multi-channel power amplification;

Speaker: sounding equipment, important and expensive;

TV: The room should be large enough (the best viewing distance is 2.4-3.6 times the diagonal length of the TV; the distance between the rear row and the wall is preferably more than 1 meter.)

Audio and video signal sources: mainly DVD players.

(1) Simple component matching

Space size: If the space is less than 20 square meters, it is advisable to use small and medium-sized speakers, and if it is more than 20 square meters, it is best to use large speakers.

Coordination with the power amplifier: impedance matching (speaker impedance should not be less than the rated load impedance of the amplifier), power matching (the rated power of the amplifier can be greater than the rated power of the speaker, but less than the rated power of the speaker), damping matching (the damping coefficient refers to the speaker impedance and The ratio of amplifier output impedance should generally be between 10-100).

(2) Introduction to common nouns

Stereo system: It is a system composed of several microphones, transmission paths and loudspeakers. After proper arrangement, it can make the listener feel that the sound source is distributed in space. What we usually call stereo combination (including home theater combination) playback is to imitate natural natural stereo through the playback of sound combination. The more similar the imitation, the better the combination.

Dolby Surround Sound: Dolby Surround System is an analog three-channel (left, right, rear surround) stereo surround system developed by Dolby Corporation of the United States. The frequency response of the rear surround channel is generally only 100-7000Hz.

Dolby Pro Logic Decoded Surround Sound: It is the second generation surround sound system developed by Dolby Company in the United States. It has been widely used in the film industry and home theater for many years, and it is still the most widely used surround sound system so far. It has four channels (left, right, center, surround channel), but before loading into the carrier (disc, etc.), the signals of the four channels need to be stored in two channels through a special encoding method. When playing, it is restored to four channels through a dedicated directional logic encoder. Now it is the mainstream of domestic home theater. The production of Dolby Pro Logic decoders or AV power amplifiers is licensed by Dolby, and the certification mark is used on the product panel. Pay attention to this mark when purchasing to avoid wrong purchases.

Center mode: Dolby Pro Logic decoding surround sound adds a center channel to ensure the authenticity of the character dialogue from the screen. Appropriate control of the center channel can adapt to the requirements of different occasions. There are three control modes: phantom, normal, wide mode. Phantom mode is for use without a center speaker. The center channel signal is evenly distributed to the left and right main channels, which can save the center speaker. The effect is relatively poor, but it is better than Dolby Surround. Normal mode is used when the power of the center speaker is less than that of the left and right channel speakers. The bass below 100Hz is only sent to the left and right channels, which is suitable for smaller audio-visual rooms and less powerful center speakers and center channel amplifiers. is the most common way. Wide mode means that the center channel uses the same full-band playback as the left and right channels, and the setting method is similar to that of a movie theater. The effect is ideal, but the center speaker needs to have a wide frequency response, a large volume, and a large listening room area. The power of the center channel is basically the same as that of the left and right channels. The three-channel mode uses the left, right and center channels, which lacks a sense of envelopment, but has a wider stereo sound field effect than ordinary two-channel playback.

Dolby AC-3: the abbreviation of Audio Coding-3, that is, audio coding 3, Dolby's third-generation audio sensory coding system. It is characterized by full digital processing, and its separation and hearing indicators are far superior to Dolby Pro Logic and THX systems. It consists of 5 full-range channels and 1 subwoofer channel (20-120Hz), which is commonly known as 5.1 soundtrack.

DTS: A coding system co-produced by American DTS Company and Universal Company, which adopts high bit and low compression ratio, less original signal loss, and creates the best surround sound effect at present. In addition, it is superior to AC-3 in terms of continuity, fineness, broadness, and hierarchy.

THX: Tomlinson Hlolmnn, chief engineer of the audio department of Lucas Films in the United States, proposed a complete set of standards on how to achieve the "X" characteristics of ISO2969 in stereo theaters. The THX system is an improvement on Dolby Surround Sound, which mainly solves the difference in listening effect caused by the different areas of the family room and the movie theater, making the sound effect of the home theater closer to the effect of the theater. On the basis of Dolby Pro Logic surround sound technology, 4 new circuits are added for subwoofer electronic subdivision, re-equalization, timbre matching, and de-correlation interference, and it has a complete 6-channel output. The requirements for room volume, acoustic conditions, sound-absorbing performance and audio equipment are high, and it is difficult for ordinary families to meet the requirements.

DSP technology: It is to use digital signal processing technology to simulate the sound effects of various hall environments. We know that when listening to the same program in different halls, the sound effect we finally hear will be different due to the different room area, geometric shape and the reflection coefficient of the wall to the sound. For example, in a large concert hall, we feel that the sound is grand and deep; in a church, the sound will become deep, solemn and mysterious; the sound of a stadium is very open, and so on. DSP technology uses electronic circuits to imitate acoustic characteristics such as reflection delay and reverberation in time, so that we can enjoy the sound effects of different environments without leaving home. Common DSP presence effect modes are: HALL (auditorium), CHURCH (church), STADIUM (stadium), LIVE (live), CONCERT (concert), CLUB (club), THEATER (theatre) and so on.

Equalization: Divide the signal into several different frequency bands, which are respectively boosted or attenuated, so that users can fine-tune the frequency characteristics of the audio source signal according to their needs. The main function is to compensate for the influence of human ears, listening environment, equipment, etc. on music or to create specific effects. Multiple modes can be individually set or preset.

BBE: It is a "high-definition original sound system" developed by BBE Sound Ine Company in the United States. It can intelligently correct and restore the signal loss or phase deviation of any audio system due to various reasons, and the clarity, sense of positioning and brightness are all improved. BBE technology is generally not sold as a separate product, but only as a supplementary part of other overall products.

3D Surround: It is a simulated surround sound system. The stereo signals of the left and right channels, after digital signal processing, pass through the left and right speakers to produce a three-dimensional surround sound field effect. Even using general binaural to create a three-dimensional surround sound field is better than stereo, but there is a considerable gap with the coded surround system. The common ones are SRS, Q-Sound, Spatialaizer, etc.

(5) Environmental conditions and placement

Living room: The size should be as large as possible, preferably larger than 15 square meters, with corresponding TV sets, speakers, etc.; the ratio of the three sides of the listening room should not be taken as an integer ratio, and it is best to conform to the golden ratio, such as "1.61:1:0.61"; The back of the speaker and the ceiling are reflective to strengthen the signal; sound-absorbing materials are used to adjust the reverberation time of the room; sound insulation is good; it can isolate the interference of external light (dark, empty, quiet)

Audio-visual electronic equipment: the upper edge of the TV is at eye level, and the distance is appropriate;

(6) Speaker placement:

Center speaker: close to the center of the screen, a small speaker can be placed on the top of the TV;

Left and right channel speakers: the width of the screen is smaller than the distance between the left and right channel speakers;

The left and right channels and the center speaker are at the same distance from the best viewing position in front of the screen (that is, arc-shaped); if there is no center speaker, the left and right speakers are slightly closer to the screen, but pay attention to antimagnetic; the vertical height of the front speaker is best kept Consistent, the treble unit is at the same level as the listener's ear; the speaker should not be placed in the corner of the wall as much as possible, it will produce resonance sound, and the position away from the wall is about 20cm;

Surround channel speakers: The correct placement should be to install the surround speakers above the two sides of the seat, hit the back wall or sideways, and use the reverse effect of the wall and the back wall to break up the directivity of the surround channels. Increase the dispersion of the surround channel (in other words, what you hear is not just two surround speakers, but the sound reflected from the entire rear, side and ceiling walls, such a large sound field, of course There is a sense of envelopment); the installation height should be about 180 cm; an experimental method can be used to make it impossible to hear which speaker the surround sound is from at the best listening position; the delay time can be adjusted appropriately and increased, which is equivalent to moving the speaker Far;

Subwoofer: no directionality; avoid resonance, damage sound quality, avoid symmetrical center positions and room openings (such as aisles, doors, windows, etc.); according to the "reciprocity theorem", the location can be determined experimentally.

Three, the function of the power amplifier

Amplify the music signal and push the speaker to sound; process and control the music signal, and control the entire playback system.

(1) Type of power amplifier

Ordinary two channels;

common five-channel(true,false);

With decoding (Dolby Pro Logic, Dolby AC-3, DTS, etc.);

(2) Evaluation indicators

Rated power (nominal): The power value that can be sustained for a long time under specified conditions, the larger the better;

Frequency Response: Refers to the relationship between the output and input signal frequencies. Whether the frequency response range is wide enough, whether the amplification is flat and consistent;

Total distortion: less than 1%, the smaller the better;

Signal-to-noise ratio: The ratio of output signal to noise, the bigger the better. Method: unplug the input plug, adjust the high, medium and low tone and volume to the maximum, in a quiet environment, you should not hear the "rustling" sound and other noises from the speaker at a distance of 1 meter from the speaker;

Dynamic range: When amplifying without distortion, the maximum and minimum amplitude range of the output signal, the bigger the better, if it is not enough, it will be distorted, such as when playing a symphony, there is no thick and pleasant feeling, and the sound is "hairy";

Separation: The degree of isolation of different channels, the bigger the better. Only input one channel, the sound of the other speaker should be as small as possible;

(3) Selection Guide

Type: Choose the power amplifier that suits your needs.

Power: There should be enough room, mainly related to speaker sensitivity, room size and listening environment, etc. The lower the speaker sensitivity, the larger the room, and the stronger the environmental sound absorption (teach more furniture, carpets, etc.), the more power required big. Usually, for a room with an area of about 18m2, the input electric power needs to be 10W if there is enough loudness for most people's appreciation requirements. Considering the requirements of high-fidelity listening, the rated power should be more than 10-20 times the power required for actual listening. The surround channel is about 20-30% of the main channel.

Distortion: less than 1%

Frequency response: 20 to 20KHz.

Function: DSP, equalization, radio, etc.

Port selection: It should be able to connect all possible program sources.

Performance: one is the sound quality, and the other is the degree of separation. When identifying sound quality, focus on listening to the effect of ordinary two-channel stereo, and use CD records. For the separation, especially the separation of the center channel to other channels is more important, otherwise, the sound image will be inaccurate. You can find a segment with only dialogue, and the other channels should be very small. Generally, the sound of the center channel can be clearly judged if it is more than 1.5 meters away from the front speaker.